Dc-dc converters are used to convert one voltage to another voltage level. Applications for dc-dc converters include converting dc power from a battery to a lower voltage to power circuits in a portable electronic device, e.g., cell phone.
FIG. 1 shows the basic topology of a conventional LLC series resonant converter for converting a voltage at its input Vg to another voltage Vo at its output. The LLC series resonant converter comprises an inverter, an LLC series resonant tank, and a rectifier. The inverter is realized by a half-bridge inverter with switches Mp and Mn that are driven complementary to generate a square wave at the input of the resonant tank. The resonant tank comprises an inductor Ls, a series capacitor Cs, and an inductor Lp connected in series to realize resonance. The inductor Lp is in parallel with the load. The rectifier comprises a center-tapped transformer, a filter capacitor CF, and two rectifier diodes DP and DN. The rectifier rectifies the ac waveform from the resonant network into a dc output. In low output voltage high current applications, the two rectifier diodes must be replaced with synchronized rectifiers (SR) to reduce the voltage drop (conduction losses) across each semiconductor rectifier.
The LLC series resonant converter has the following features: natural zero voltage switching at both the inverter and rectifier sides, wide load range, limited switching frequency swing, and fast transient response, all of which closely approximate the required characteristics of the desired dc-dc converter. However, the large circulation currents in LLC series resonant converters limit their application. The large circulation currents place high stress on the semiconductor switches, capacitor and inductors of the resonant tank, requiring the converters to use higher tolerance components. Further, the large circulation currents increase conduction losses, reducing the efficiency of the converters.
Therefore, there is a need for dc-dc converters that preserve the desirable features of LCC series resonant converters while overcoming their limitations.